No need to worry about gravity at the atomic-/nano-scale

When a metal is grown onto a substrate of itself (homoepitaxy), the growth front is typically smooth, or at most is roughened by the formation of shallow hills (called surface mounds). The underlying reason for the roughening has been recognized to be of kinetic nature: Atoms landed on an upper terrace do not have enough time to overcome the "road blocks" provided by the steps and fill all the valleys (known as the Villian instability). In a recent set of experiments, it was observed that very "tall" (yet still only tens of nanometers in height) hut-shaped monuments can be formed when only a few nanometers of Al films are grown on an Al(110) substrate (Mongeot et al., Phys. Rev. Lett. 91, 016102 (2003)). The faceting instability responsible for the formation of the hut-shaped nanocrystals was attributed to true upward migration of the Al atoms, a generic phenomenon expected to exist for many of the fcc metal (110) homoepitaxial (and even heteroepitaxial) growth systems (Zhu et al., Phys. Rev. Lett. 92, 106102 (2004)). A brief highlight on this notion of "atoms going uphills" could be found at (Fichthorn and Scheffler, Nature 429, 617 (2004)).